I just got back from South Carolina... the best I got down and back was just under 19mpg, towing about 2000#s and around 600#s on the bed. To be fair, I ran winter fuel, studded snow tires (more rolling resistance), and about the same weight, on almost the same trip and averaged better than 20mpg.
While it does make it quieter, it didn't seem to affect mileage, and may have even caused it to drop a bit. I'm going to give it these three gallons before I decide to continue running it. On a side note, it does smoke (under hard acceleration of lugging) worse with the 2-cycle.